1. Field of the Invention
This invention relates to a generator system for internal combustion engines which converts rotational energy into electrical energy and, more particularly, to a generator system for an internal combustion engine, which can optimize a driving torque and a generated power (energy) of a generator by generating a rotating magnetic field in a polyphase winding of a rotor regardless of an internal combustion engine speed, that is, a rotation speed of the rotor.
2. Description of the Related Art
A generator system for vehicles or ships includes an alternator (AC generator; ACG) with its rotating shaft coupled through an alternator belt to a crankshaft of an engine; a rectifier for converting AC power into DC power, the AC power generated by the alternator according to the engine speed; and a regulator for controlling the voltage level of the DC power according to the voltage at a battery.
FIG. 9 is a schematic diagram showing a structure of a conventional alternator 50, in which a DC field coil 53 is wound around a rotor 52 integrally formed with the rotating shaft and a three-phase coil 55 is wound around a stator 54. In the condition that the DC field coil 53 is excited by supplying direct current from the battery (not shown), the rotor 52 is rotated to create an alternating magnetic field. Consequently, the three-phase coil 55 of the stator 54 generates AC power with a frequency corresponding to the rotational speed of the rotor 52. Such a conventional alternator is an AC generator using a synchronous motor. In some cases, the rotor 52 may be provided with a permanent magnet instead of the DC field coil 53.
In recent vehicles, power consumption has increased because of current tendencies to electrify or electronically control vehicle parts including the engine and to widely use audio and navigation systems. This requires the alternator to increase the capacity of power generation. The electric power generated by the alternator is reduced as its rotation speed drops. It is therefore necessary to increase the ratio of the alternator speed to the engine speed so that sufficient generated power can be obtained even at low engine speeds such as 1000 r.p.m or less. In many cases, the pulley ratio is set twice or more for that purpose.
On the other hand, high velocity revolution and low idling of the engine is accelerated as the engine output and efficiency increase. For this reason, the ratio of the minimum and maximum engine speeds is becoming larger. When the pulley ratio is set higher for the electric power required at low engine speeds, the alternator speed tends to exceed the maximum rate determined from mechanical restrictions in high velocity revolution. In the case where a pulley ratio is set to a higher value, a power generated by the alternator is greater than the maximum power required, during high rotation of the engine. For this reason, a problem has arisen such that wiring and couplers or the like with larger capacity must be used.
On the other hand, in the case where the pulley ratio is set to a lower value so as to meet the desired generated power, during high rotation of an engine, a sufficient power is not generated during a lower rotation. For this reason, a problem has arisen such that a battery is excessively discharged because a power consumption due to an electric load becomes greater than the generated power.
As described above, in the generator system for an internal combustion engine which is operated at variable speed like an engine for vehicle, there has arisen a problem that it is difficult to properly set a pulley ratio in the light of durability of the alternator, capacity of wiring and the couplers or the like, and charge and discharge of the battery.
In order that the rotation speed of alternator is arbitrarily set regardless of the engine speed, for example, in Japanese Patent Publication No. Sho 62-33465, there has been proposed the mechanism for mechanically varying a pulley diameter of an alternator driving pulley. However, in the case where the aforesaid mechanical speed change mechanism is employed, a problem has arisen such that the structure or construction becomes complicate and is large in size.
In an internal combustion engine for vehicle and the like, when an electric load such as a headlight and an air conditioner having a much power consumption is turned on/off, in response to the electric load variation, an excitation power of a direct current field coil 53 is controlled in order to increase/decrease a generated power. As a result, an engine torque (herein after, referred simply to as "driving torque for alternator") required for driving the alternator varies, and then the rotation speed of engine varies. In particular, when a heavy electric load is changed from an off state to an on state and the driving torque suddenly increases, the rotation speed of engine is lowed. For this reason, if the engine is in an idling state, an engine stall (stop) may be caused, or if a vehicle is in a running state, the engine becomes in a kind of braking state and the driverability becomes worse.
In order to solve the above problems, for example, in Japanese patent Laid-open Publication No. Hei 1-277650, there has been proposed a control unit which makes a decision whether or not an electric load is applied, and opens a throttle valve so as to make high a preset value of an engine speed, if a decision is made that the heavy electric load is applied. Further, in Japanese Patent Laid-open Publication No. Hei 5-180047, there has been disclosed a control unit which controls a duty ratio of a field current supplied to a field coil of a stator in response to an increase and decrease in an electric load. According to the aforesaid conventional techniques, a driving torque for an alternator varies in accordance with an increase or a decrease of the electric load; for this reason, there has arisen a problem that a large amount of load is applied to an alternator belt, or no stability is obtained in the engine speed because quick control thereof is not performed.